1. Field of the Invention
The present invention relates to an apparatus for evaluating the density (lightness/darkness) and evenness of density of a pattern printed on an article, more particularly to an apparatus for detecting the density of printed patterns, for example, printed letters and marks, and then evaluating the evenness thereof.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), etc., information consisting of, for example, a model code, a manufacturing lot number, and a manufacturing date is normally printed on an outer surface of a housing just after manufactured or before shipping so as to identify the IC device, etc. The printing is normally effected by an automatic printing machine or a stamp. The information is essential for discrimination of devices in testing and assembly into printed circuit boards as well as for controlling the quality of lots of such devices. Consequently, the information must remain legible over a long period.
Many apparatuses have been proposed to inspect the information printed on the housing, particularly to investigate the density of the printed information and the evenness thereof. If an IC device is printed with insufficient density or evenness of information, that device may be detected and rejected before, for instance, testing or shipping.
A paper "A Machine Vision System for Inspection of Keyboards" by J. Wilder, appearing in Signal Processing, May 1983, pages 413 to 421, deals with a system which, as a major task, can verify that each location on a keyboard contains correct, properly oriented key and that the graphics are not badly distorted. The system functions to process shading (density) by executing filtering, intensity measurement, edge extraction, feature extraction, and multi-frame averaging. In order to realize the above processing, the system includes image-input means, means for converting the input image into binary-coded image data, and means for comparing the binary-coded image data with a predeterminal reference pattern stored in a memory.
The paper, however, does not disclose the signal processing to an extent enabling understanding by others. In addition, the system is of an absolute type wherein the reference pattern is previously fixed. As a result, the system does not operate stably in the face of changes in illumination or noise. It is also difficult and complicated to determine the fixed reference pattern to obtain reliable results.
Japanese Unexamined Patent Publication (Kokai) No. 55-13453 discloses an apparatus for inspecting the print on an article monitoring a printing ribbon and a printing head in a cash dispenser, etc. The apparatus includes an image-input device, a discriminating-converting device, counting means, a memory device, and first and second judgement means. The discriminating-converting device discriminates input image signals with a plurality of reference levels and converts the discriminated signals to coded signals to obtain a plurality of discriminated level signals corresponding to the density of the printed pattern. The counting means counts the number of discriminated levels in a horizontal sweep direction, i.e., X direction to eliminate noise and/or needless data, and thereby, enable reduction of the memory size. The first judgement means verifies effective discriminated level signals. The second judgement means compares the effective discriminated level signals with a predetermined reference pattern to output either a "good" or "bad" signal. The apparatus is also of an absolute type, however, so suffers from the same disadvantages as mentioned above.
Japanese Unexamined Patent Publication (Kokai) No. 55-157078 discloses a method for finding a faulty pattern. The method includes the steps of extracting a plurality of positional data by shifting input image data by certain increments with respect to a center portion (pixel) to be measured; comparing the positional data with a predetermined reference pattern; and outputting either a "good" or "bad" signal. The method is an absolute type, so has the same disadvantages set forth above.
Japanese Unexamined Patent Publication (Kokai) No. 58-168185 discloses an apparatus for judging a mark applied on an outer surface of an electric bulb which can detect deposition of dirt, partial erasure, and uneven density of marks thereon. This also suffers from the same disadvantages.